In a way, U2 can't be overrated. They are too widely known and people have the information and have placed them where they are. That's not a very fun answer though.
I'm not a U2 guy, but I get it. I think they're rated about right. They certainly appealed to people. They did things a little differently, kind of consolidating some different sounds into an appealing package. You can't really argue their talent, quality, and if you're deeper in the woods you could probably argue their uniqueness but in the scope of popular music it's pretty clear. I've been told by many that they're really great live. The bulk of their songs may not "move" me, but in their prime they were better than most stuff you came across in pop culture. Like I said, I'm not a big fan, but I think they're where they should be.